The invention generally relates to a housing device for mounting an assembly of press-pack semiconductor devices and associated heat sinks, and to a stacked an assembly of assembly of such devices.
Medium to high voltage semiconductor devices such as gate turnoff thyristors (GTOs), silicon controlled rectifiers (SCRs) and insulated gate bipolar transistors (IGBTs) are used in a variety of power system applications. For example, IGBTs may be used as the switching elements in a power inverter bridge controlling a 1200 horsepower motor. These mediums to high power semiconductor devices are characterized by current and voltage ratings of approximately 100 to 300 amps and 1.2 to 10 kV.
Due to the relatively high power capacities of such semiconductor devices they have to be packaged so as to contend with a number of issues, including heat dissipation, electrical contact characteristics and arcing. One common form of packaging used by the manufacturers of such devices is the xe2x80x9cpress packxe2x80x9d. In this packaging structure the semiconductor material is enclosed in a typically cylindrically-shaped casing. The tubular body of the casing is constructed out of an electrically non-conductive material; ceramic is often used for its durability at high temperatures. The tubular body is capped with electrically conductive metallic plates which function as some (or all) of the terminals of the semiconductor device, such as the anode and cathode of a GTO or thyristor. These terminal end-faces present relatively broad planar surfaces for enabling good electrical and thermal contact with other power circuit components such as electrically conductive heat sinks. To further ensure good electrical contact and meet other operating requirements, press pack devices require a pre-specified amount of pressure to be applied thereto, typically in the range of 2-20 kN, although much higher forces are also possible.
The pressure or mounting force applied to the press-pack devices is provided by some sort of clamping mechanism. A typical clamping mechanism comprises two threaded rods fitted with plates for applying pressure provided by clamping nuts. Vice-like clamping mechanisms can also be used. These clamping mechanism are also often used to stack multiple numbers of press-pack devices and heat sinks together in abutting relationship. The resultant assembly, or xe2x80x9cstackxe2x80x9d, can be used in a variety of power circuits such as the leg of an inverter and minimizes the number of clamping mechanisms required, which are extraneous elements of the power circuit.
In assembling the stack the conventional practice is to axially align all of the elements thereof in order to ensure uniform application of the mounting force. The way this was accomplished in the prior art is through the use of small guide pins inserted into locating holes formed on the abutting faces of the press-pack devices and heat sinks. Many press-pack devices are manufactured with small holes situated in the centre of the terminal end-faces thereof for this purpose. However, a significant problem arises with this system when it is necessary to replace one of the press-pack devices in the field. More specifically, the heat sinks of the stack are typically quasi-rigidly mounted to a supporting structure such as a housing or cabinet and therefore capable of moving apart only a few thousands of an inch to allow for thermal expansion. This distance is considerably less than the length of the guide pin as disposed in the locating hole. So, to replace one press-pack device in a large stack often meant the whole stack had to be removed from the cabinet, disassembled to replace the press-pack device, then re-assembled and re-installed. This task could require well over an hour. Alternatively, field personnel would attempt to bypass the disassembly procedure altogether by trying to pry out a press-pack device from the stack through the use of sheer force. This usually resulted in a significant scarring or gouging of the terminal end-faces of the press-pack device caused by dragging it over the embedded locating pins, and the gouges were often significant enough so as to render the press-pack devices inoperative because of a change in the thermal transfer characteristics.
A further limitation of conventional stack assemblies is that they do not readily accommodate the installation and removal of a press-pack semiconductor device which is mounted onto a printed circuit board.
The invention seeks to overcome various limitations of the prior art by providing a housing have compartments for mounting a predetermined arrangement of heat sinks and printed circuit boards (xe2x80x9cPCBsxe2x80x9d) carrying press-pack semiconductor devices. The housing also includes a compartment for accommodating a force application member which provides the necessary mounting force required by the press-pack devices. The housing receives and distributes this force amongst the foregoing elements.
According to one aspect of the invention, a stack assembly is provided which includes the following components: one or more heat sinks, one or more PCBs, each having a press-packaged semiconductor device mounted therein, a plate having a force applying member, and a housing having compartments for accommodating the aforementioned component in a predetermined abutting arrangement. The compartments accommodating the heat sinks and PCBs are sized slightly larger than the heat sinks and the PCBs to allow each such component a predetermined amount of horizontal play in its corresponding compartment. Each PCB includes a bracket for mounting the PCB onto a corresponding heat sink. The mounting bracket and the size of the heat sink compartments are configured to substantially axially align the press-pack devices with a longitudinal axis defined by the force applying member.
In the preferred embodiment, the compartments for accommodating the heat sinks are provided with locating nubs for positioning the heat sinks. The width between locating nubs is slightly larger than the width of a corresponding heat sink, thereby providing the heat sink with a predetermined amount of horizontal play in its compartment.
In the preferred embodiment, the rear wall of each heat sink compartment is formed with at least one longitudinal receiving slot for receiving a heat sink anchor. The heat sink anchor features a body, which is fitted in the slot and has a breadth or width smaller than the length of the slot so as to be able to slide therein. Two flanges of the anchor respectively abut opposite sides of the rear wall so as to retain the anchor to the housing. The anchor also features a bore for enabling the heat sink to be fastened thereto. In this manner the anchor is floatingly mounted to the housing and does not interfere with the horizontal play afforded to the heat sink by the excess width between the locating nubs.
In the preferred embodiment, a flexible connector is used to connect a heat sink to a power source. The flexible connector includes a power lead and a bus bar. The power lead is connected at one end to a heat sink and at the other to the bus bar. The bus bar is rigidly connected to a terminal. To permit play in the heat sink in at least the horizontal direction, the bus bar is curved.
In an alternative embodiment, the flexible connector includes: a power bar with a transverse non-threaded bore, a bus bar with a slot, and at least one spacer with a longitudinal bore. The spacer is inserted through the non-threaded bore in the power lead and through the slot in the bus bar. A bolt is passed through the bore in the spacer and secured with a nut. The spacer is longer than the combined thickness of the power lead and bus bar thereby permitting play in the power lead and heat sink connected thereto. A wire connected at one end to the power lead and at the other to the bus bar ensures good electrical contact between the power lead and bus bar.
In the preferred embodiment, a recess in the heat sink compartment furthest from the force applying member accommodates a reaction plate which bears against the housing and distributes the clamping force indirectly applied to it by the force applying member through the sequence of axially aligned stack elements.
In the preferred embodiment each heat sink compartment has a rear wall having an opening of approximately the same shape and area of the rear face of a heat sink. A gasket is installed between the heat sink and the rear wall to form an airtight seal there between.
The housing also features compartments for accommodating resistor networks. Each resistor network compartment is provided with registering slots for engaging the edges of horizontal locator plates that are used to hold a resistor network in place in the compartment.
In the preferred embodiment, one form of heat sink anchors provide a means to connect the stack assembly to a power supply lead. These anchors have a flange for abutting the housing and a shaft connected at one end to the flange. The shaft has a diameter sized smaller than the length of the longitudinal slot in the rear wall so as to slide therein. A clip fits into a circumferential groove of the shaft so as to permit the anchor to be floatingly mounted to the housing through the slot. The anchor also features a smooth longitudinal bore which mediates the passage of a bolt between a heat sink and a power lead having a threaded bore, thus tightly securing the heat sink to the power lead via this anchor.